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Upgrade of Neutron Energy Spectrometer with Single Multilayer Bonner Sphere Using Onion-like Structure

  • Received : 2015.07.17
  • Accepted : 2016.09.06
  • Published : 2016.09.30

Abstract

Background: In order to measure neutron energy spectra, the conventional Bonner Sphere Spectrometers (BSS) are widely used. In this spectrometer, several measurements with different size Bonner spheres are required. Operators should, therefore, place these spheres in several times to a measurement point where radiation dose might be relatively high. In order to reduce this effort, novel neutron energy spectrometer using an onion-like single Bonner sphere was proposed in our group. This Bonner sphere has multiple sensitive spherical shell layers in the single sphere. In this spectrometer, a band-shaped thermal neutron detection medium, which consists of a LiF-ZnS mixed powder scintillator sheet and a wavelength-shifting (WLS) fiber readout, was looped to each sphere at equal angular intervals. Amount of LiF neutron converter is reduced near polar region, where the band-shaped detectors are concentrated, in order to uniform the directional sensitivity. The LiF-ZnS mixed powder has an advantage of extremely high light yield. However, since it is opaque, scintillation photons cannot be collect uniformly. This type of detector shows no characteristic shape in the pulse height spectrum. Subsequently, it is difficult to set the pulse height discrimination level. This issue causes sensitivity fluctuation due to gain instability of photodetectors and/or electric modules. Materials and Methods: In order to solve this problem, we propose to replace the LiF-ZnS mixed powder into a flexible and Transparent RUbber SheeT type $LiCaAlF_6$ (TRUST LiCAF) scintillator. TRUST LiCAF scintillator can show a peak shape corresponding to neutron absorption events in the pulse height spectrum. Results and Discussion: We fabricated the prototype detector with five sensitive layers using TRUST LiCAF scintillator and conducted basic experiments to evaluate the directional uniformity of the sensitivity. Conclusion: The fabricated detector shows excellent directional uniformity of the neutron sensitivity.

References

  1. Bramblett R, Ewing R, Bonner T. A new type of neutron spectrometer. Nucl. Instrum. Methods. 1960 Oct;9(1):1-12 . https://doi.org/10.1016/0029-554X(60)90043-4
  2. Koontz P, Keepin G, Ashley J. ZnS(Ag) phosphor mixtures for neutron scintillation counting. Rev. Sci. Instrum. 1955;26(4):352-356. https://doi.org/10.1063/1.1771295
  3. Sugimoto D, et al. Neutron TOF experiments using transparent rubber sheet type neutron detector with dispersed small pieces of LiCaAlF6 scintillator. Physics Procedia. 2014;60:349-355. https://doi.org/10.1016/j.phpro.2014.11.047
  4. Watanabe K, et al. Wavelength-shifting fiber signal readout from Transparent RUbber SheeT (TRUST) type LiCaAlF6 neutron scintillator. Nucl. Instrum. Methods Phys. Res., Sect. A. 2015 Jun;784: 260-263 https://doi.org/10.1016/j.nima.2014.11.109
  5. Yanagida T, et al. Europium and sodium codoped LiCaAlF6 scintillator for neutron detection. Appl. Phys. Express. 2011 Sep; 4(10): 106401 1-3. https://doi.org/10.1143/APEX.4.106401
  6. Sato T, et al. Particle and heavy ion transport code system PHITS, Version 2.52, J. Nucl. Sci. Technol. 2013;50(9):913-923. https://doi.org/10.1080/00223131.2013.814553